docs: def _lookup_global(self, symbol):
          """Helper for lookup_symbol that only looks up global variables.
  
          Args:
            symbol: Symbol
          """
          assert symbol.parts
          namespace = self.namespaces
          if len(symbol.parts) == 1:
              # If there is only one part, look in globals.
              namespace = self.namespaces[None]
          try:
              # Try to do a normal, global namespace lookup.
              return self._lookup_namespace(symbol, namespace)
          except Error as orig_exc:
              try:
                  # The normal lookup can fail if all of the parts aren't
                  # namespaces. This happens with OuterClass::Inner.
                  namespace = self.namespaces[None]
                  return self._lookup_namespace(symbol, namespace)
              except Error:
                  raise orig_exc